The Collaborative International Dictionary of English v.0.48:

Reinforcement \Re`in*force"ment\ (-ment), n.
   See Reenforcement.
   [1913 Webster]

WordNet (r) 3.0 (2006):

reinforcement
    n 1: a military operation (often involving new supplies of men
         and materiel) to strengthen a military force or aid in the
         performance of its mission; "they called for artillery
         support" [syn: support, reinforcement, reenforcement]
    2: information that makes more forcible or convincing; "his
       gestures provided eloquent reinforcement for his complaints"
       [syn: reinforcement, reenforcement]
    3: (psychology) a stimulus that strengthens or weakens the
       behavior that produced it [syn: reinforcing stimulus,
       reinforcer, reinforcement]
    4: a device designed to provide additional strength; "the
       cardboard backing was just a strengthener"; "he used gummed
       reinforcements to hold the page in his notebook" [syn:
       strengthener, reinforcement]
    5: an act performed to strengthen approved behavior [syn:
       reward, reinforcement]

Moby Thesaurus II by Grady Ward, 1.0:

252 Moby Thesaurus words for "reinforcement":
   Pavlovian conditioning, accelerando, acceleration, accession,
   accessory, accompaniment, accouterment, addenda, addendum,
   additament, addition, additive, additory, additum, adjunct,
   adjunction, adjuvant, advocate, affirmation, affixation,
   agglutination, aggravation, alpenstock, amplitude, annex,
   annexation, antinode, appanage, appendage, appendant, appurtenance,
   appurtenant, arm, armament, athletic supporter, attachment,
   attestation, augment, augmentation, authentication, auxiliaries,
   back, backbone, backing, backing up, bandeau, bearer, bearing out,
   beefing-up, behavior, blowing up, blowup, bolster, bolstering, bra,
   brace, bracer, bracing, bracket, brassiere, buttress, buttressing,
   cane, carrier, case hardening, catering, certification, cervix,
   chandlery, circumstantiation, coda, complement, concentration,
   concomitant, condensation, conditioned response, conditioning,
   confirmation, consolidation, continuation, corollary,
   corroboration, corroboratory evidence, corset, crest, crook,
   crutch, de Broglie wave, deepening, diffraction, documentation,
   electromagnetic radiation, electromagnetic wave, endowment,
   enhancement, equipment, exacerbation, exaggeration, explosion,
   extension, extrapolation, finding, fitting out, fixture, forces,
   fortification, foundation garment, frequency, frequency band,
   frequency spectrum, fulcrum, furnishing, furnishment, girdle,
   guided wave, guy, guywire, hardening, heating-up, heightening,
   in phase, increase, increment, information explosion,
   instrumental conditioning, intensification, interference,
   investment, invigoration, jock, jockstrap, joining, junction,
   juxtaposition, light, logistics, longitudinal wave, magnification,
   mainstay, maintainer, mast, mechanical wave, men, neck,
   negative reinforcement, node, offshoot, operant conditioning,
   out of phase, outfitting, pendant, period, periodic wave,
   personnel, pickup, population explosion, positive reinforcement,
   prefixation, preparation, procurement, proof, prop, providing,
   proving, proving out, provision, provisioning, psychagogy,
   purveyance, radio wave, ratification, ray, redoubling, reeducation,
   reflex, refreshment, reinforce, reinforcements, reinforcer,
   reinvigoration, relief, reorientation, replenishment,
   reserve forces, reserves, resonance, resonance frequency, rest,
   resting place, restrengthening, resupply, retailing,
   revivification, rigging, seismic wave, selling, shock wave,
   shoulder, shroud, side effect, side issue, soldiers, sound wave,
   speedup, spine, sprit, staff, standing rigging, stave, stay,
   step-up, stick, stiffener, strengthener, strengthening,
   subsidization, subsidy, substantiation, subvention, suffixation,
   superaddition, superfetation, superjunction, superposition,
   supplement, supplementation, supply, supplying, support, supporter,
   supporting evidence, surface wave, sustainer, tailpiece, tempering,
   tidal wave, tightening, transverse wave, trough,
   unconditioned reflex, undergirding, uniting, upholder, validation,
   verification, victualing, walking stick, wave, wave equation,
   wave motion, wave number, wavelength